In a valve gated injection molding apparatus, a valve pin reciprocates through a nozzle to open and close a mold gate forming an opening to a mold cavity. Improper alignment of the valve pin with the mold gate may result in leaking at the mold gate, which may cause blemishes around the gate area of the molded part. Further, improper valve pin alignment may result in damage to or pre-mature wear of the valve pin and/or the mold gate. Such damage necessitates frequent repair or replacement of the valve pin and/or mold gate components, which can be costly.
Solutions for improving valve pin alignment have typically included a guide positioned towards the downstream end of the nozzle melt channel to capture and align the free end of the valve pin before it exits the nozzle and enters the mold gate. Because melt is required to flow past the guide when the valve pin is in the open position, a plurality of circumferentially spaced slots are typically provided in either the valve pin or the guide for melt to move around the valve pin and valve guide. A disadvantage of this arrangement is that the melt flow in the nozzle melt channel must separate to pass around the guide means, and subsequently reunite downstream of the guide, which may cause weld lines to appear in the molded product. Furthermore, having a guide in the nozzle melt channel typically causes melt stream color changes in the injection molding apparatus to be less efficient.
Further, many injection molding applications include a plurality of hot runner nozzles that are arranged closely with one another, i.e., having a tight pitch. These hot runner nozzles may have melt channels with very small diameters. Valve pin gating presents several challenges in small diameter hot runner nozzles. For example, the valve pin diameter must be sized to allow sufficient space for the melt to flow between the pin and the melt channel. If the flow restriction is too great, the injection pressure required must be increased and, as a result, the shear on the melt stream may cause overheating and degradation of the melt material. However, a valve pin that would not cause a significant flow restriction would likely be weak and subject to damage during use due to its extremely small diameter.
Solutions for improving a nozzle channel of a small diameter nozzle include carving a groove into an exterior of the valve pin to create a nozzle melt channel defined between the nozzle channel inner surface and the groove of the valve pin. However, these grooves do not define clear melt channels since the valve pins move with respect to the nozzle channel. Often these grooves do not provide symmetric heat distribution to the melt channel because the channel is not positioned along an axis of the nozzle, which can cause uneven temperatures in the melt.